The present disclosure relates to an image forming apparatus which includes a plurality of paper feed devices that are stacked.
Examples of an image forming apparatus include a multifunctional peripheral, a copying machine and a printer. The image forming apparatus includes a paper feed device. The paper feed device stores sheets which are used for printing. A user sets the sheets used for printing in the paper feed device. The paper feed device supplies the sheets one by one at the time of printing. There are paper feed devices which can set a plurality of sheets. However, there is an upper limit on the number of sheets which can be set in the paper feed device (the thickness of a sheet bundle). When the number of sheets set exceeds the upper limit, an error such as a paper jam or a non-paper feed jam is more likely to occur. The following technology on the excessive loading of sheets in a paper feed device is known.
Specifically, an image forming apparatus is disclosed which detects that a transfer member loading portion is raised to an upper limit position so as to perform an operation of feeding a transfer member, which includes overload determination means for the transfer member and which does not perform the feeding operation when an overload state is detected. In this configuration, paper bending or a paper jam is attempted to be avoided which easily occurs when the transfer members are loaded on the transfer member loading portion beyond a paper feed limit.
In an image forming apparatus, a plurality of paper feed devices (paper feed units) may be attached. In an image forming apparatus, as an optional device, a paper feed device may be additionally provided. A plurality of paper feed devices are attached, and thus it is possible to increase the number of sheets stored. In general, the sheets of only one size can be set in one paper feed device. When only one paper feed device is present, and a sheet which is stored differs in size from a sheet which is desired to be printed, it is necessary to replace sheets. However, the sheets of different sizes are stored in a plurality of paper feed devices, and thus it is not necessary to replace sheets.
When a plurality of paper feed devices are attached, in terms of space saving, the paper feed devices may be stacked. For example, a plurality of paper feed devices are stacked in a lower portion of an image forming apparatus. In each of the paper feed devices, a paper feed roller (pickup roller) which feeds out the uppermost sheet loaded is provided. The sheet fed out by the paper feed roller is transported toward an image formation unit. A sheet which is supplied from the paper feed devices other than the lowermost paper feed device joins a transport path of the sheet which connects the lowermost paper feed device to the image formation unit.
In an image forming apparatus, a plurality of paper feed rollers may be rotated with one motor. In other words, when the motor is rotated, all the paper feed rollers are rotated. In such an image forming apparatus, in only a paper feed device which performs paper feeding, a placement plate (plate on which sheets are set) is raised. Then, the paper feed roller is brought into contact with the uppermost sheet. In this way, paper feeding is performed only by the selected paper feed device.
On the other hand, a user may overload a paper feed device with sheets. When sheets are set so as to exceed a specified number, the uppermost sheet may be brought into contact with the paper feed roller without the placement plate being raised. Disadvantageously, in the image forming apparatus in which all the paper feed rollers are rotated when the motor is rotated, not only the paper feed device which performs paper feeding but also an overload paper feed device performs paper feeding.
When a plurality of paper feed devices individually perform paper feeding, the sheets which are fed from the paper feed devices collide with each other (overlap each other). Thus, a jam may occur. Printing may be performed on the sheet of an unintended size. Hence, in the image forming apparatus in which all the paper feed rollers are rotated when the paper feed motor is rotated, it is necessary to determine which one of the paper feed devices encounters overload on sheets so as to prevent the occurrence of such a problem. However, a special sensor for detecting an overload state before the start of paper feeding is provided, and thus the manufacturing cost of the image forming apparatus and the paper feed devices is increased.
The known technology described above is not a technology which is intended for a plurality of paper feed devices. The known technology is also not a technology on the image forming apparatus in which all the paper feed rollers are simultaneously rotated when one paper feed motor is rotated. Hence, even with the known technology, it is impossible to solve the problem described above. In the known technology, an upper limit detection mechanism portion which detects that a tray (transfer member loading portion) on which sheets are placed is raised to the upper limit position is used as the overload determination means. Hence, since in the known technology, the upper limit detection mechanism portion does not detect that the paper feed roller reaches the upper limit, when sheets are excessively loaded such that the sheets are brought into contact with the paper feed roller, it is impossible to detect the excessive loading of the sheets.